Known methods and apparatus for determining the amount of dirt or the number of scratches on a photographic element's surface such as a negative or slide require either a visual inspection of the negative or slide and manual recording of the data or the making of a photographic print followed by the visual inspection of the print and the manual recording of the data. This procedure is very labor intensive and time consuming in the first case, and very labor intensive, time consuming, and wastes paper in the second case.
A patent of interest for its teaching in this art is U.S. Pat. No. 4,189,235, entitled "Test Device For Dynamically Measuring The Degree of Dirt Accumulation On Bank-Notes" by Guter et al. This patent describes a method for inspecting opaque web materials for dirt accumulation. The reflected signal from a light source is sensed by three adjacent photosensors which scan the material. When the signal from the center sensor is significantly different then the side sensors the local region is classified as dirt. Materials with greater than a predetermined rate of detected dirt regions are rejected. Potential problems with this invention include sensitivity to noise in the reflected signal due to either normal variations or system noise which makes it difficult to set robust thresholds on the difference signals and the restriction to sensing in a linear array of three sensors.
In digital scanning systems it is not possible for the scanner to distinguish between the real photographic image and surface artifacts such as dirt and scratches. Therefore, the artifact is scanned and digitized as part of the photographic image. When the scanned image is written to memory or written as hard copy, the artifact, which was not part of the photographic image, is also written along with the photographic image itself. In conventional optical printing systems, if dirt or a scratch appears on the photographic image, the photographic negative or slide may be cleaned and the print remade or the print itself maybe retouched. In the case where the images are written to an optical disc, it is not convenient nor desirable to rewrite the image, because many images are written onto a disc before the images are viewed, and once an image is written it cannot be removed. Therefore it is necessary to monitor the dirt present in the photofinishing environment and to assess the effectiveness of film cleaning methods.
Another patent of interest is U.S. Pat. No. 4,677,680, entitled "Method And Device For Inspecting Image" by Harima et al. In this patent a method is described for inspecting a moving web media for defects. The density values of local regions in the imaged medium are compared with a predetermined true image of that region. Differences between the true and imaged signals are compared to a threshold. This method has the potential problems of requiring an integral registration step between the true and sensed images in addition to adding complexity this step does not compensate for differences in spatial illumination variations across the medium or random noise differences.
Yet another patent of interest is U.S. Pat. No. 5,033,095, entitled "Scanning Image Analyzer For Accumulating Quantifiable Contaminants of Webs" by J. J. Marcantonio. This patent teaches a system for inspecting test sheets of web material e.g. paper and reporting the results to an operator via a display. The pixel brightness values of a scanned image are thresholded to create a pixel map of detected dust. contiguous dust regions on this map are calculated and a report of the number and size of the dust regions is displayed to the operator. The key difficulty with such a system is again the determination of suitable signal thresholds to provide robust detection in the presence of spatial variations in illumination and of system noise. In the application of the system to the scanning of film transparencies a fair amount of the contaminating dirt is partially transparent and generates only a slight signal difference from that of the uncontaminated medium. The system also places constraints on the quality of both the test target and the scanner with regard to minimizing spatial variations in brightness, because of the use of a single threshold across the image.
The present invention solves the aforementioned problems with an apparatus and a method for determining the amount, size, shape, and location of the artifact. In particular, the invention describes a robust method for setting a spatially continuous series of thresholds across the image which are insensitive to spatial brightness variations in the test target or the scanner illumination and sensor and which allows the detection of contaminants whose brightness signal overlaps the local brightness distribution of the background medium. The present invention is extremely operator friendly in that in that normal operation requires no user adjustments of the parameters. The present invention also allows for unique adjustment of the detected dust region map to compensate for marginal coverage of sensor areas by dirt fragments and visually insignificant breakages of large dirt fragments.